I was doing some thinking about guild raids recently and how we have more than enough for two, but not quite enough for three. It occurred to me how much simpler organizing raid would be if the two sizes were 10 and 15 (or even 10 and 20) rather than 10 and 25.
10 and 25 man raids already give the same rewards, so the only reason to run one over the other is a matter of how many people you have interested in raiding. Although the consensus nowadays seems to be that 25 mans aren't a convenient size. Anyway looking a normal raiding roster fluctuations, sometimes you have a lot of interest and sometimes you don't. In the times of hardship it is a struggle just to get 10 people from week to week and in the times of plenty rotations are a necessary evil.
But with 10 and 15 man raids a 10 man with a 13-14 person roster could easily pick up a few more interested players and run the 15 man version. If there are a bunch of sign outs for whatever reason, they could drop back down to the 10 man version. If there was so much interest that 15 wasn't even enough the raid could split and do two 10 mans which in turn could expand into 15 mans of their own or recombine back into a 15 man. Basically some combination of 10 and 15 could accommodate smooth roster size transitions of any magnitude.
25 mans would still make sense for LFR with role ratios being what they are.
The more I think about 15 man raid vs 25 man the more I like the idea. It solves the whole issue of 25 mans being harder to organize. It allows raid rosters to be more flexible and not exclude people just because there are only 10 spots. It doesn't introduce issues like moving from 40 mans to 25 mans or 25 mans to 10 mans where many people would be unable to raid with the same people after converting. Serious 25 man raid could splint into a 10 man A team and 15 man B team or vice versa.
15 man raid seems an elegant solution to the problem where 10 mans are too small and 25 mans are too big.